1. Field of the Invention
The present invention relates generally to SuperContinuum (SC) light, and more particularly, to methods and apparatus for producing the SC light for medical and biological applications.
2. Description of the Related Art
Light is one of nature's most important and versatile phenomena. In a manner similar to that of a courier, light can transfer information from one point to another, and in a manner similar to that of an alchemist, it can alter matter and stimulate chemical reactions. More specifically, light can initiate and moderate key processes in chemistry, biology and condensed matter.
The versatility of light is a direct result of the many forms it is able to take, such as, for example, brief flashes, focused spots, broad continuous beams, dim or intense light, polarized light, low- or high-frequency light, and light containing many frequencies at once. The frequency of visible light determines its color, and is related to the light's wavelength, i.e., shorter wavelengths correspond to higher frequencies.
An incandescent bulb emits light across a full visible spectrum, resulting in white light. However, light from an incandescent bulb has several drawbacks. Specifically, this type of light has a relatively low intensity and brightness. The light from an incandescent bulb is also neither coherent nor collimated in a single direction. Therefore, the individual light particles, or photons, do not oscillate in phase with one another. Lasers do not have the above-mentioned drawbacks that result from light of an incandescent bulb. However, instead of emitting white light, a laser emits a narrow band of frequencies, resulting in light of a specific color. For many applications, coherent light at a single frequency, or a narrow band of frequencies, is more than adequate. However, having a light source, such as the SC, which combines the properties of a laser with those of a broad bandwidth incandescent bulb, provides for a new realm of applications.
Seminal work on the SC's generation was performed when it was discovered in 1970. See, R. R. Alfano et al., PRL, 24, 592-594, 584-587, 1219-1222, (1970). 100 MegaWatt (MW), 10 picosecond (ps) pulses were focused into condensed materials in order to produce the SC with a white light continuum of colors. The SC light can be generated over a frequency octave using microstructure fibers, holey filters, and photonic crystal fibers, and using modest energies of <100 femtosecond (fs) ps lasers. See, K. J. Ranha et al., Opt. Lett. 25, 25 (2000), and S. Coen, Chau, Leonhardt, and J. Harvey, JOSA B, 26, 753 (2002). Using kilowatt peak power fs pulses, SC spans from 400 to 1600 nanometers (nm) can be generated in photonic crystal fibers. For example, 1 meter of NonLinear-Polarization Maintaining (NL-PM) 750 photonic crystal fiber made by Crystal Fibre Corp. can produce more than an octave, i.e., 1200 nm bandwidth using 800 nm 50 fs −67 milliWatt (mW) average power from a Ti-sapphire laser. The broad SC spectrum results from Self-Phase Modulation (SPM), 4 Wave Mixing (4WM), and stimulated Raman and Soliton generation. Thus, the SC light can be generated on a spectrum greater than an octave, where 500 to 1000 nm is an octave, and a two octave SC spans from 400 to 1600 nm.
A bright SC beam can be produced by focusing ˜350 microJoule (μJ) 70 fs pulses from Ti-sapphire laser systems into a long metallic cylinder of 90 centimeters (cm), which contains rare gases, such as, for example, Argon (Ar), Krypton (Kr) and Xenon (Xe), at modest pressures from 2 to 30 Atmospheres (atm). See, P. B. Corkum et al., PRL, 57, 2268 (1986). The nonlinear parameter, n2, for Ar is 9.8×10−20 cm2/W atm, for Kr is 2.8×10−19 cm2/W atm, and for Xe is 8.2×10−20 cm2/W atm. See, Lehmeier et al., Opt. Comm., 56, 67-72 (1985).
A milliJoule (mJ), which is the energy required to lift a paper clip several centimeters against the earth's gravity, may appear to be a small amount of energy. However, when a mJ is packed into a ps and focused into a tight spot it represents a GigaWatt (GW) of power and an extremely high intensity. With this high intensity, the pulses can propagate through a few centimeters (cm) of glass, inducing a Kerr effect strong enough to spread the pulses' bandwidth considerably even in the short time that they passed through the glass by distortion of the electron clouds in the material.
Fibers used for SC generation are known as microstructure fibers. A cross section of such fibers reveals a pattern of holes that runs continuously through the entire length of the fibers. In one commonly used design, the pattern of holes surrounds a solid silica core, similar to a honeycomb with only the central hole filled. The core has a high index of refraction, whereas the surrounding cladding, with its air holes interspersed with silica, has a lower refraction index. The concentric arrangement of refractive indices serves to guide the light pulses along the fiber. The use of these fibers with zero and anomalous dispersion, has enabled the generation of the SC light extending more than two octaves from InfraRed (IR) to UltraViolet (UV). The placement of the zero dispersion point in the fibers in blue and Near InfraRed (NIR) will produce pulses covering UV, visible, and NIR regions.
The generation of the SC light in optical fibers has unleashed a wide range of applications. One of the most important and mature of these applications is the development of extremely accurate frequency measurements and clocks. The SC light is useful in optical frequency comb techniques, which enable improved accuracy with simpler and smaller systems. Specifically, self-referencing becomes possible when the frequency comb extends across a full octave. In this approach, the frequency of light is doubled at the low-frequency end of the spectrum and is used to interfere with light at the high-frequency end.
Researchers are now striving to develop systems capable of measuring frequencies to a fractional accuracy of 10−16 to 10−18. Such extreme accuracy would have practical implications for improvements in Global Positioning Systems, space navigation, and the alignment of very large arrays of radio telescopes. The systems would also be utilized in tests of special relativity and related fundamental principles such as the isotropy of space, the symmetry of matter and antimatter, and the constancy of the constants of nature.
Frequency measurements and clocks are two facets of the same technology. Ultimately, the optical frequency comb might enable fractional accuracies of 10−18, which would be ideal for timing in optical computers and even for detecting oil and mineral deposits by their minute effects on the nearby gravitational field. The SC light is also enabling technology to produce shorter pulses into attosecond (10−18 sec) and zeptosecond (10−21 sec) regions.
An application with more immediate commercial implications than ultra-precise frequency measurements is telecommunications. Several of the SC's key properties make it an ideal basis for telecommunication systems that are capable of transmitting data more than 1,000 times faster than present-day systems. Optical fiber carrying IR light is already the most widely used means of sending data at high rates over long distances. In an effort to keep up with the ever-increasing worldwide demand for larger-capacity communications systems and networks, there is a need to include more data into a fiber. The goal is to achieve transmission rates of terabits (1012) and pentabits (1015) per second. Typical fiber-optic systems currently transmit data between cities at about 10 gigabits per second, or 0.01 terabit per second.
The ultrabroad bandwidth of the SC light makes it a cost-effective way to obtain numerous wavelength channels without having to use hundreds of lasers. That bandwidth could be utilized in superdense wavelength division multiplexing, in which data streams are encoded onto many different wavelengths of light that are transmitted simultaneously. The SC, unlike the light from 100 individual lasers, can be coherent across a wide range of frequencies, which aids in the degree of control that can be brought to bear on the light.
Alternatively, a series of ultrashort pulses of the SC light (shorter than 100 fs, or 10−13 second) can be sent, with sequences representing different data channels interleaved with one another, referred to as Time-Division Multiplexing (TDM). With short pulses, it is important to be able to control the precise relation between the individual oscillations of the electric field (the carrier wave) and the pulse envelope. This property, referred to as the relative phase of the carrier and the envelope, determines, for example, whether the peak of the pulse envelope occurs at an instant when the electric field of the wave is at a peak or a trough, or somewhere in between. The properties of the SC light facilitate such control.
Data transmission rates of terabits/second have already been achieved using a small segment of the SC light spectrum. However, many challenges remain in order to improve the speed and achieve petabit/second operation. These challenges include reducing the duration of a bit to about a ps and increasing the number of coherent wavelengths in the SC.
The telecommunication applications rely on producing the SC light in the completely controlled environment of an optical fiber; however, for some applications the SC light is generated in open air. One such application is the remote sensing of molecular species present in air. When intense ultrafast laser pulses travel through the air, they can produce long, narrow “filaments” in which the air is ionized. Within those filaments electrons are knocked off the air's molecules, forming a plasma of positive ions and negatively charged electrons. These filaments can guide the light pulses and keep them from spreading, a process that scientists attribute to a balance between defocusing caused by diffraction (the tendency for a wave to spread out from a small aperture) and self-focusing caused by the ionized plasma.
Within the filaments a significant amount of the pulses' power can convert to SC white light over distances greater than 20 meters. Pollutants and aerosols in the air will absorb the light at characteristic frequencies, and the broad spectrum of the SC light enables one to detect their absorption spectra simultaneously in the UV, visible and IR bands.
In addition to probing the air around us, the SC light is useful in producing high-resolution images of tissues within us. Optical Coherence Tomography (OCT), can be carried out in situ in living organisms as a diagnostic tool to measure tissue layers.
To produce an OCT image, a light is split into two parts. A first part of the light illuminates a spot in the sample, whereas a second part, or a reference light, enters a length of fiber. When the reference light recombines with light that the sample reflected or scattered, the two interfere strongly, provided that they each spent the same length of time on their respective journeys. High-resolution OCT imaging relies on a short coherence length of the source light, which requires a very accurate timing match.
Thus, when the spot of light penetrates into the sample, only light coming back from one specific depth will interfere with the reference light. Scanning the light laterally across the sample while keeping the travel time of the reference light fixed thereby produces a two-dimensional image of the sample at a certain depth. The thickness of the layer that contributes to the image is called the axial resolution of the image.
Early OCT imaging systems relied on a type of diode to provide the light and had an axial resolution of 10 to 15 microns. The axial resolution also depends on the bandwidth of the light source. A broader bandwidth enables finer resolution. The SC light has a short coherence length and a bandwidth broader than any fs laser, making it ideal for high-resolution OCT imaging. The SC light generated in microstructured fibers has been used to produce images of cells with an axial resolution of 0.5 micron.
Light is also capable of photo-activating molecular components within tissue (in the matrix and/or cells) in order to fuse a cut together with minimal scarring using the water absorption from overtone and combination vibrational bonds at 1450 nm, spanning from 1000 to 1600 nm. Tissue welding can be achieved using lasers, such as tunable Cr4+ lasers, semiconductor lasers, and fiber lasers, where the weld strength follows the absorption spectrum of water. Tissue wounds, bruises, and burns can be healed using laser and lamp light covering UV to visible regions at an average irradiance of ˜100 mW/cm2. Microsecond pulsed lasers from 1850 nm to 2100 nm may be used to stimulate nerves, and to kill bacteria and viruses by exciting upper UV states with UV and blue light transitions.